1. Field of the Invention
The present invention is directed to a method of cooling blow molded plastic articles by injecting a liquid into the interior of the freshly molded article. More specifically, the invention relates to a method of blow molding an article and subsequently cooling the molded article by injecting a liquid into the interior portion of the article and then applying a positive gas pressure to remove the liquid from the interior of the article.
2. Description of the Related Art
U.S. Pat. No. 3,937,610 (""610) teaches a method of cooling the interior of a blow molded article with a liquid mist. The ""610 patent is directed to a method for reducing the cycle time of molding a blown article by reducing the time needed to cool the article before it can be removed from the mold. After the article is blown, small globules of liquid are circulated through the interior of the article. The liquid evaporates when it contacts the hot plastic wall, thus reducing the temperature of the blown article. Air is circulated within the article to evaporate any remaining liquid. A volume of cooling liquid is not circulated within the interior of the article. Only small liquid globules suspended within the mist contact the article walls. While the method described in the ""610 patent reduces the time needed to cool the article, the cycle time reduction is not as great as in the present invention. The liquid globules suspended within the mist do not have sufficient capacity to rapidly cool thick-walled blow molded articles when compared to a solid liquid medium.
Two additional references which also teach including liquid within the blowing medium are U.S. Pat. No. 5,498,390 and Japanese Patent Application No. 05-023094. These references similarly require that the cooling liquid be suspended within the blowing medium and evaporate upon contact with the hot parison walls. While this method of manufacturing blow molded articles reduces the cycle time when compared with ambient cooling, it does not provide the rapid cooling of the present invention due to the superior ability of liquid water to remove heat energy.
Japanese Patent Application No. 05-104616 (""616) teaches a method of making a container by blow molding. After the container has been blow molded, a cooling liquid is introduced into the interior of the molded article. This is known as xe2x80x9cwater castingxe2x80x9d where a quantity of cooling liquid equal to the volume of the container is introduced and held within the article. The amount of heat removed from the article is limited by the volume of water. In the present invention, the cooling liquid is circulated through the article and more than the container volume of cooling liquid may be circulated through the article to remove more heat. The present invention also introduces a constant xe2x80x9cin-outxe2x80x9d flow of water, creating turbulence. Turbulent flow inside the article provides the greater thermal cooling.
The ""616 patent application teaches a method of molding a container around a blowing mandrel. The parison seals between the outer perimeter of the water inlet and the mold to form a fluid-tight seal. Sealing between the parison and the water inlet system has been a key impediment for using liquid cooling for blow molding. Most mold applications do not support the use of a mandrel between the mold walls. In these applications, needle or pin is used to pierce the parison and inject cooling liquid into the molded article. Forming a fluid-tight seal between the parison and the needle is necessary to implement liquid cooling.
The present invention is useful for applications where it is not feasible to place a stationary water inlet between the mold halves. These and other disadvantages of the prior art are overcome by the method taught in the present invention.
The present invention is directed to a method of blow molding a plastic article. A plastic article is blow molded within a mold. The article has a plastic parison wall. The wall is pierced by a first and a second needle or blowing pin. The wall forms a fluid-tight connection between the first needle and the parison wall. A volume of cooling liquid is injected into the article through the first needle. The cooling liquid cools the interior wall. Gas is vented from within the article through the second needle while the cooling liquid is injected into the article. The cooling liquid is evacuated through the first needle after the interior wall is cooled sufficiently to be removed from the mold.
The invention enables the rapid manufacture of blow molded articles by reducing the dwell time needed within the cavity for the molded article to solidify. The liquid cooling removes heat from the blow molded article faster than cryogenic gases, such as nitrogen or carbon dioxide, and faster than an evaporative mist. The invention enables the use of liquid cooling for articles that cannot be manufactured by placing a water inlet between the mold halves. The use of a blowing pin or needle enables the placement of the water inlet and outlet in the uppermost part location which is often not at the mold parting surface. The needles pierce the parison and form a fluid-tight seal between the needle and the article wall to prevent the cooling liquid or evacuation gas from leaking into the mold.
A combination of valves, pump and accumulator enables the cooling liquid to be quickly flooded into the interior portion of the article and removed by pressurized gas. The first needle is placed at a location that easily enables all of the water within the article to be drained. This usually places the first needle at the lowest portion of the mold. A positive air pressure is introduced through the second needle to force out all of the cooling liquid. The article is removed from the mold with little or no remaining cooling liquid therein.
These and other objects, features, and advantages of the present invention will become more readily apparent when viewed in connection with the accompanying drawings wherein like reference numerals correspond to like components.